Enterovirus infections are usually subclinical but cause also various kind of diseases. Typical enterovirus diseases are meningitis, paralysis, myocarditis, generalized infections in newborns, hand, foot and mouth-disease, herpangina, pleurodynia, hepatitis, rash, exanthemas and respiratory diseases including pneumonia. In addition, enterovirus infections have been suspected to play a role in the pathogenesis of dilated cardiomyopathy, atherosclerosis, postviral fatique syndrome and Type 1 diabetes mellitus.
The group of enteroviruses includes a total of 64 different serotypes. Polioviruses are the most widely known enteroviruses including 3 different serotypes (poliovirus types 1, 2 and 3) which all can cause meningitis and typical paralytic poliomyelitis (flaccid paralysis). Meningitis is frequently caused by several non-polio enteroviruses, which are the most common cause of aseptic meningitis. Myocarditis is caused mainly by coxsackie B serotypes but also other enterovirus serotypes may be involved. Hand, foot and mouth-disease is mainly caused by certain coxsackie A serotypes and severe infections of infants are related to coxsackie B serotypes. Paralytic diseases can also be caused by some other serotypes than poliovirus serotypes. The serotypes related to atherosclerosis and Type 1 diabetes are not known. In type 1 diabetes the most suspected ones have been coxsackieviruses B4 and B5 but also other than coxsackie B serotypes may be involved.
The only enterovirus vaccine, which has been used in human beings is poliovirus vaccine. This vaccine includes all three poliovirus serotypes and gives effective prevention against paralytic poliomyelitis. The protection is based on the induction of neutralizing antibodies, against these serotypes and is serotype specific. Thus, neutralizing antibodies, which are induced by poliovirus vaccines do not protect against any other enterovirus serotypes than the three poliovirus serotypes. The role of T-cell mediated immune responses in the protection against poliovirus infections is not known. The generally accepted view is that they play only a minor role while antibodies are more important in the elimination of infection and in the protection against reinfections.
Two different types of poliovirus vaccine have been developed. The killed inactivated poliovirus vaccine (IPV; Salk vaccine) includes formalin-inactivated polioviruses (all 3 serotypes). This vaccine is given parenterally using subcutaneous injections. It induces a Th2-type immune response characterized by strong antibody response and high levels of neutralizing antibodies against all poliovirus serotypes and gives effective prevention against paralytic poliomyelitis. However, it induces only weak local immune response in the gut. As gut associated lymphoid tissue is the primary replication site of polioviruses, IPV vaccine can not protect against poliovirus infection but only against the complications of infections. IPV can induce only weak cytotoxic T-cell immune responses.
The other poliovirus vaccine is oral poliovirus vaccine (OPV; Sabin vaccine) which includes live attenuated polioviruses (all three serotypes). This vaccine is given per os and the virus replicates in the same way as the wild polioviruses in the body. As the vaccine is given per os in the same way as natural enterovirus infections are acquired, it induces strong local immunity in the intestine, which prevents from later poliovirus infections. Thus, OPV vaccinated individuals usually do not become infected by polioviruses because the virus is not able to replicate in the intestine. The nature of this protection is not completely understood but it probably depends on both neutralizing antibodies and T-cell mediated immunity. OPV induces stronger T-cell responses than IPV and it induces mainly Th1-type T-cell responses characterized by strong cytotoxic T-cell responses.
Vaccines against non-polio enteroviruses are not available for human use. The reason is that the large number of enterovirus serotypes makes it difficult to make a pan-enterovirus vaccine and, on the other hand, the serotypes, which are causing the most severe non-polio enterovirus diseases, are highly variable. Myocarditis and cardiomyopathies have been associated with coxsackie B group viruses, meningitis and neonatal infections with several different serotypes and practically nothing is known about the serotypes possibly related to the development of atherosclerosis. In Type 1 diabetes the responsible serotypes are not known except that polioviruses are not involved. The general view is that poliovirus vaccines should not be effective in the prevention of Type 1 diabetes or other non-polio enterovirus diseases, but that the prevention of non-polio enterovirus diseases would require new vaccines which should induce neutralizing antibodies against the serotypes to be protected. Another reason for the lack of human non-polio enterovirus vaccines is that the safety of such vaccines has not been reliably confirmed. Thus, there is no effective vaccine or any other treatment for the prevention of non-polio enterovirus diseases in man.
Inactivated and subunit vaccines which include certain coxsackie B viruses have been tested in animal models. They have induced good antibody levels in mice and rabbits and effectively protected from infections caused by the serotypes which were included in the vaccine (Fohlman et al., 1990 and 1993; See and Tilles, 1994 and 1997). However, these vaccines have not been tested in human beings. The main reason for this is that the current knowledge on the mechanisms of immune protection against enteroviruses is limited and the safety of such vaccines can not be guaranteed. The safety issue has become very important after the discovery of the unexpected side-effects related to the use of inactivated respiratory syncytial virus (RSV) and measles vaccines in humans (Fulginiti et al., 1967; Harris et al., 1969; Kapikian et al., 1969). These vaccines paradoxically increased the severity or modulated the course of natural infections. The most probable explanation for these adverse effects is that these kind of inactivated vaccines generally induce good antibody response but very poor cytotoxic T-cell response. Thus, they may have induced a shift towards Th2-type antibody mediated immunity which resulted in the atypical symptoms. This indicates the need for very detailed data on the effect of the vaccine on the course of natural infections and careful evaluation of the safety issues.
The advantage of the immunization regime of the present invention is that it is based on the oral poliovirus vaccine (OPV) which has been extensively used in almost all countries of the world and which has proved to be very safe and effective. The poliovirus vaccines are actually one of the most effective and safest vaccines ever developed and have led to an almost complete eradication of poliovirus infections from the world. The only clinically relevant complication of OPV is the risk of vaccine associated paralysis. However, its frequency is extremely low (about 1 per 1-10 milj, vaccinees).
The advantage of the immunisation regime of the present invention is that it is based on the oral poliovirus vaccine (OPV) which has been extensively used in almost all countries of the world and which has proved to be very safe and effective. The poliovirus vaccines are actually one of the most effective and safest vaccines ever developed and have led to an almost complete eradication of poliovirus infections from the world. The only clinically relevant complication of OPV is the risk of vaccine associated paralysis. However, its frequency is extremely low (about 1 per 1-10 milj. vaccinees).
The general view is that immunity against enterovirus infection is based on the presence of neutralizing antibodies against the virus. These antibodies can efficiently neutralize the virus when it enters the body. The significance of neutralizing antibodies is reflected by the fact that patients who have abnormally low levels of antibodies due to an immune deficiency are particularly susceptible for enterovirus infections. Neutralizing antibodies can be detected for prolonged periods after the infection. They contribute to the eradication of the virus during primary enterovirus infection and protect against reinfections. However, they can not protect against infections, which are caused by other serotypes. Thus, the protection by these antibodies is serotype specific. Accordingly, it is generally thought that it is essensial for the efficacy of enterovirus vaccines that the vaccine is able to induce high titres of neutralizing antibodies against the serotypes which should be protected. The only currently used enterovirus vaccine is poliovirus vaccine which includes all three poliovirus serotypes.
The present invention is based on the finding that, in contrast to the general paradigm, oral poliovirus vaccines could also protect against other enterovirus infections than poliovirus infections and could therefore be used for the prevention of various non-polio enterovirus diseases, which have been described in detail in previous paragraphs, and diseases where the role of enteroviruses has been suspected including Type 1 diabetes mellitus, chronic fatigue syndrome and atherosceloris. This protection would be based on efficient induction of T-cell responses and local mucosal immunity by repeated OPV vaccinations. T-cell immune responses are known to cross-react between certain enterovirus serotypes when analysed in vitro by T-cell proliferation assay (Beck and Tracy, 1990; Graham et al., 1993). However, it was not known whether this cross-reactivity had any biological significance in vivo. It was not either known to what extent T-cell responses which are induced by OPV vaccinations can cross-react with non-polio enteroviruses and whether this had any clinical relevance.
We have previously evaluated these questions by analysing enterovirus specific T-cell responses in young infants. We found that some infants, who had never experienced any coxsackievirus B infection according to the lack of neutralizing antibodies, had strong T-cell proliferation response against purified coxsackievirus B4 antigen, which probably reflects the cross-reactivity of T-cells which have initially been induced by other enterovirus infections (Juhela et al., 1998). In addition, polio vaccination at the age of 6 months induced stronger T-cell response to purified coxsackievirus B4 and poliovirus antigens in children who had serological evidence of previous enterovirus infection compared to children who had no previous enterovirus infections (Juhela et al., 1998). This suggests that T-cells can cross-react between polioviruses and non-polio enteroviruses.
Our aim is to utilise this T-cell cross-reactivity by priming cross-reactive T-cell memory using OPV vaccinations. This, in turn, would make the immune responses to other enteroviruses stronger and more rapid (secondary-type response) and in this way speed up the eradication of the virus during acute non-polio enterovirus infections. OPV can not totally protect from these infections as it does not induce neutralizing antibodies against non-polio enteroviruses but it may protect against viremia and severe illnesses by potentiating the T-cell responses by inducing cross-reactive memory T-cells. This kind of T-cell help can potentiate both the production of neutralizing antibodies during infection as well as cytotoxic T-cell responses against non-polio enteroviruses. It may also booster antibodies against other enteroviruses than the serotype causing the acute infection by eliciting anamnestic immune responses. Induction of anamnestic responses means that OPV stimulates memory T-cell clones, which have originated from previous enterovirus exposures and in this way leads to their activation and induction of antibodies against all these serotypes. This kind of anamnestic response is used in the present regime to enhance enterovirus antibody levels in pregnant women thus providing protection for their infants.